A display system usually includes an illuminating module and a control module. The illuminating module includes one or more light sources, e.g., multiple light-emitting diode (LED) strings. The control module which may include a micro-controller, a video processor and an audio processor controls the on/off and dimming of the illuminating module and processes the video and audio signals. The power requirement of the illuminating module and the power requirement of the control module may be different. Thus, an input AC voltage is converted to a first DC voltage to power the illuminating module and a second DC voltage different from the first DC voltage to power the control module.
FIG. 1 illustrates an example of a conventional display system 100. An AC/DC converter 104 receives an AC voltage from an AC power source 102 and outputs a DC voltage VIN. A transformer 130 receives the DC voltage VIN at a primary winding 106, generates an output voltage VOUT1 at a first secondary winding 110, and generates an output voltage VOUT2 at a second secondary winding 108. The output voltage VOUT1 is used to power a control module 128 which includes a micro controller, a video processor and an audio processor. The output voltage VOUT2 is used to power an illuminating module 126 which includes multiple LED strings. The control module 128 generates an ON/OFF signal to turn on or turn off the illuminating module 126, and generates a dimming signal DIM to adjust the brightness of the illuminating module 126. An error amplifier 118 senses VOUT1 through a voltage divider 120 and controls an optocoupler 116 to generate a feedback signal FB indicative of VOUT1. A DC/DC controller 114 receives the feedback signal FB and generates a pulse signal to control a switch 112 which is coupled in series with the primary winding 106. By controlling the switch 112, the power delivered from the primary winding 106 to the secondary winding 110 is adjusted such that VOUT1 is regulated to a first level to satisfy a power requirement of the control module 128. By controlling the switch 112, VOUT2 also varies. A power converter, e.g., a boost converter 122, is coupled between the secondary winding 108 and the illuminating module 126. The boost converter 122 regulates VOUT2 to a second level to satisfy a power requirement of the illuminating module 126. Thus, in order to generate the output voltage VOUT2 which has a different voltage level from the output voltage VOUT1, an extra power converter (e.g., the boost converter 122) is used, which increases the cost of the system.
FIG. 2 illustrates another example of a conventional display system 200. Elements labeled the same as in FIG. 1 have similar functions. The conventional display system 200 includes a first transformer 230 and a second transformer 232. The first transformer 230 generates a first output voltage VOUT1 to power a control module 128. The second transformer 232 generates a second output voltage VOUT2 to power an illuminating module 126. A first DC/DC controller 214 controls a first switch 204 in series with a primary winding of the first transformer 230 to adjust the output voltage VOUT1 based on a feedback signal FB1 from a first optocoupler 236. A second DC/DC controller 216 controls a second switch 202 in series with a primary winding of the second transformer 232 to adjust the output voltage VOUT2 based on a feedback signal FB2 from a second optocoupler 234. Therefore, an extra DC/DC controller 216, an extra transformer 232 and an extra optocoupler 234 are used, which also increase the cost of the system.